sick note conversion, implementation

* using SickNoteConversion class
* handling complex cases like start and/or end dates of sick note and
  application are differing

refs #14633
sick note conversion, simplified

* only full sick note can be converted to vacation now
* added field active to SickNote entity
* if sick note is converted to vacation the active field is set to false
* only SickNotes with active=true can be edited or converted
** adjusted frontend not to show edit and convert buttons for inactive
   sick notes
** adjusted sick note list to show inactive sick notes in another text
   color
** adjusted SickNoteController: editing and converting only possible
   with active sick notes, else error jsp is returned

refs #14633
sick note list, print css

inactive sick notes won't be displayed for printing

refs #8755
sick note conversion, added mail notification

refs #14633